Once the DD-WRT was flashed, the ath1 (5G) was only connecting at 130 Mbs. Where as the stock firmware was able to connect a a rate of 300 Mbs. After poking around a while, the following settings worked and I was able to connect at 300 Mbs.

In the Advanced settings for wireless on the DD-WRT GUI, select Wireless Network Mode as N Only (5 GHz)